Are Active Galactic Nuclei the Solution to the Excess Cosmic Radio Background at 1.4 GHz?

TL;DR

This paper investigates whether active galactic nuclei (AGN) and their host star formation can explain the excess cosmic radio background at 1.4 GHz, concluding they account for less than 60%, implying unknown sources or evolution.

Contribution

The study combines an X-ray background synthesis model with empirical radio to X-ray ratios to quantify AGN and star formation contributions to the CRB at 1.4 GHz.

Findings

AGN and host star formation contribute less than 9% to the CRB.

Known sources account for less than 60% of the CRB.

Additional unknown sources or evolution are needed to explain the excess.

Abstract

Recently the ARCADE 2 experiment measured the cosmic radio background (CRB) and found the brightness temperature of the CRB at 1.4 GHz to be ~480 mK. Integrating the flux density from the observed 1.4 GHz radio source count produces a brightness temperature of ~100 mK---less than a quarter of the observed CRB at 1.4 GHz. Radio quiet AGN are a large fraction of the 1.4 GHz uJy sources and typically host significant star formation. Thus, it is possible that AGN and host star formation could be responsible for some fraction of the excess CRB at 1.4 GHz. Here, an X-ray background population synthesis model is used in conjunction with empirical radio to X-ray luminosity ratios to calculate the AGN contribution to the CRB at 1.4 GHz including the emission from host star formation. It is found that AGN and host star formation contribute <~9% of the CRB at 1.4 GHz. When all known 1.4 GHz radio source classes are considered, <~60% of the CRB at 1.4 GHz is accounted for; therefore, it is necessary that either known radio sources evolve significantly at flux densities below current survey sensitivity limits or a new population of low flux density radio sources exist.